Inflammation is a complex process in which the body's defense system combats foreign entities. While the battle against foreign entities may be necessary for the body's survival, some defense systems improperly respond to foreign entities, even innocuous ones, as dangerous and thereby damage surrounding tissue in the ensuing battle.
Atopic allergy is a disorder where genetic background dictates the response to environmental stimuli. The disorder is generally characterized by an increased ability of lymphocytes to produce IgE antibodies in response to ubiquitous antigens. Activation of the immune system by these antigens also leads to allergic inflammation which may occur after their ingestion, penetration through the skin, or after inhalation. When this immune activation occurs and pulmonary inflammation ensues, this disorder is broadly characterized as asthma. Certain cells are important in this inflammatory reaction in the airways and they include T cells and antigen presenting cells, B cells that produce IgE, and mast cells/basophils that store inflammatory mediators and bind IgE, and eosinophils that release additional mediators. These inflammatory cells accumulate at the site of allergic inflammation, and the toxic products they release contribute to the tissue destruction related to the disorder.
While asthma is generally defined as an inflammatory disorder of the airways, clinical symptoms arise from intermittent airflow obstruction. It is a chronic, disabling disorder that appears to be increasing in prevalence and severity.1 It is estimated that 30–40% of the population suffer with atopic allergy, and 15% of children and 5% of adults in the population suffer from asthma.1 Thus, an enormous burden is placed on our health-care resources in the treatment of these disorders.
Both the diagnosis and treatment of asthma and related disorders are problematic.1 In particular, the assessment of inflamed lung tissue is often difficult, and frequently the cause of the inflammation cannot be determined. Although atopic asthma is an ecogenetic disorder, knowledge about the particular variant genes has only recently been discovered. Methods to detect these genetic variations and their role in inflammation, diagnosis and prognosis remain to be determined. What is needed in the art is the development of technology to expedite the diagnosis of atopic asthma that specifically relates to variation in genes responsible for susceptibility/resistance to this atopic disease.
Current treatments suffer their own set of disadvantages. The main therapeutic agents, β-agonists, reduce the symptoms, i.e., transiently improve pulmonary functions, but do not affect the underlying inflammation so that lung tissue remains in jeopardy. In addition, constant use of β-agonists results in desensitization which reduces their efficacy and safety.2 The agents that can diminish the underlying inflammation, the anti-inflammatory steroids, have their own known list of side effects that range from immunosuppression to bone loss.2 
Because of the problems associated with conventional therapies, alternative treatment strategies have been evaluated.38-39 Glycophorin A,37 cyclosporin,38 and a nona peptide fragment of IL-2,36 all inhibit interleukin-2 dependent T lymphocyte proliferation.28 They are, however, known to have many other effects.2 For example, cyclosporin is used as a immunosuppressant after organ transplantation. While these agents may represent alternatives to steroids in the treatment of asthmatics,36-39 they inhibit interleukin-2 dependent T lymphocyte proliferation and potentially critical immune functions associated with homeostasis. What is needed in the art is technology to expedite the development of therapeutics that are specifically designed to treat the cause, and not the symptoms, of atopic asthma. These therapies represent the most likely way to avoid toxicity associated with nonspecific treatment. The therapies would selectively target a pathway, which is downstream from immune functions, such as IL-2 mediated T lymphocyte activation, that is necessary for the development of asthma and which would explain the episodic nature of the disorder and its close association with allergy. Nature demonstrates that a pathway is the appropriate target for asthma therapy when biologic variability normally exists in the pathway and individuals demonstrating the variability are not immunocompromised or ill except for their symptoms of atopic asthma.
Because of the difficulties related to the diagnosis and treatment of atopic allergies including asthma, the complex pathophysiology of these disorders is under intensive study. While these disorders are heterogeneous and may be difficult to define because they can take many forms, certain features are found in common among asthmatics. Examples of such features include abnormal skin test response to allergen challenge, eosinophilia in the lung, bronchial hyperresponsiveness (BHR), bronchodilator reversibility, and airflow obstruction.3-10 These expressions of asthma related traits may be studied by quantitative or qualitative measures.
In many cases, elevated IgE levels are correlated with BHR, a heightened bronchoconstrictor response to a variety of stimuli.4,6,8,9 BHR is believed to reflect the presence of airway inflammation,6,8 and is considered a risk factor for asthma.11-12 BHR is accompanied by bronchial inflammation, including eosinophil infiltration into the lung and an allergic diathesis in asthmatic individuals.6,8,13-18 
A number of studies document a heritable component to atopic asthma.4,10 Family studies, however, have been difficult to interpret since these disorders are significantly influenced by age and gender, as well as many environmental factors such as allergens, viral infections, and pollutants.19-21 Moreover, because there is no known biochemical defect associated with susceptibility to these disorders, the mutant genes and their abnormal gene products can only be recognized by the anomalous phenotypes they produce.
The functions of IL-9 and the IL-9 receptor (the IL-9 pathway) now extend well beyond those originally recognized. While the IL-9 pathway serves as a stimulator of T cell growth, this cytokine is also known to mediate the growth of erythroid progenitors, B cells, mast cells, and fetal thymocytes.22,23 The IL-9 pathway acts synergistically with IL-3 in causing mast cell activation and proliferation.24 The IL-9 pathway also potentiates the IL-4 induced production of IgE, IgG, and IgM by normal human B lymphocytes,25 and the IL-4 induced release of IgE and IgG by murine B lymphocytes.26 A role for the IL-9 pathway in the mucosal inflammatory response to parasitic infection has also been demonstrated.27,28 
Nevertheless, it is not known how the sequence of the IL-9 receptor specifically correlates with atopic asthma and bronchial hyperresponsiveness. It is known that IL-9 binds to a specific receptor expressed on the surface of target cells.23,29,30 The receptor actually consists of two protein chains: one protein chain, known as the IL-9 receptor, binds specifically with IL-9; the other protein chain is shared in common with the IL-2 receptor.23 In addition, a cDNA encoding the human IL-9 receptor has been cloned and sequenced23,29,30 This cDNA codes for a 522 amino acid protein which exhibits significant homology to the murine IL-9 receptor. The extracellular region of the receptor is highly conserved, with 67% homology existing between the murine and human proteins. The cytoplasmic region of the receptor is less highly conserved. The human cytoplasmic domain is much larger than the corresponding region of the murine receptor.23 
The IL-9 receptor gene has also been characterized.30 It is thought to exist as a single copy in the mouse genome and is composed of nine exons and eight introns.30 The human genome contains at least four IL-9 receptor pseudogenes. The human IL-9 receptor gene has been mapped to the 320 kb subtelomeric region of the sex chromosomes X and Y.23 
In spite of these studies, no variants of the IL-9 receptor gene have been discovered. There is, therefore, a specific need for genetic information on atopic allergy, asthma, bronchial hyperresponsiveness, and for elucidation of the role of IL-9 receptor in the etiology of these disorders. This information can be used to diagnose atopic allergy and related disorders using methods that identify genetic variants of this gene that are associated with these disorders. Furthermore, there is a need for methods utilizing the IL-9 receptor variants to develop therapeutics to treat these disorders.